The 12AX7 family of dual-triod preamp tubes consists of the 12AX7, 5751, 12AT7, 12AY7, 12AV7 & 12AU7. These are all pin compatable with one another, the only differences being the gain factor of each tube. A common substitution is to replace a 12AX7 with a 5751 or a 12AT7 to tame a preamp that tends to overdrive too easily, allowing you to get a better 'clean' sound out of your amp.
| Tube | Gain | Acceptable Substitutes | |||||
|---|---|---|---|---|---|---|---|
| 12AX7 | 100 | 5751 | 12AT7 | 12AY7 | |||
| 5751 | 70 | 12AX7 | 12AT7 | 12AY7 | |||
| 12AT7 | 60 | 5751 | 12AY7 | ||||
| 12AY7 | 45 | 5751 | 12AT7 | 12AV7 | |||
| 12AV7 | 41 | 12AY7 | 12AU7 | ||||
| 12AU7 | 19 | 12AV7 | |||||
It is important to note that the above is not carved in stone and basically any of these tubes can be substituted for any other. You can always replacing a higher-gain tube with a lower-gain tube, using your ears to tell you whether this was good idea or not. As example of this, my Fender Pro Junior amp came stock with two 12AX7 tubes - one for the preamp and one for the inverter. I have since re-tubed it with a 12AU7 in the preamp stage and a 12AT7 for the inverter. The 12AU7 in the preamp gives a nice clean sound, and the 12AT7 inverter helps prevent overdriving the power amp.